Laminated retardation optical element, process of producing the same, and liquid crystal display

ABSTRACT

In a liquid crystal display  90 , a laminated retardation optical element  10  is placed between a polarizer  102 A on the incident side and a liquid crystal cell  104 , and a ˜/4 retardation film  102 C  10  is placed between a polarizer  102 B on the emergent side and the liquid crystal cell  104 . The laminated retardation optical element  10  comprises: a ˜/4 retardation layer  14  having the function of bringing, to light that passes through this retardation layer, a  15  phase difference corresponding to a quarter of the wavelength of the light; and a C plate-type retardation layer  16  that acts as a negative C plate. The ˜/4 retardation layer  14  and the C plate-type retardation layer  16  are laminated to a transparent substrate  12  in  20  the order mentioned, and are optically bonded to each other. The I/4 retardation layer  14  comprises as its main component a horizontally-aligned, cross-linked nematic liquid crystal, while the C plate-type retardation layer  16  comprises as its main component a 25 cross-linked chiral nematic liquid crystal (a cross-linked nematic liquid crystal and a cross-linked chiral agent) or cross-linked discotic liquid crystal.

This is a Continuation of U.S. patent application Ser. No. 10/655,582filed on Sep. 5, 2003 now U.S. Pat. No. 7,324,180, which claims priorityto Japan Patent Application No. 2002-261717, filed on Sep. 6, 2002. Theentire disclosure of the prior application is hereby incorporated byreference herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a laminated retardation optical elementfor use in a liquid crystal display or the like, especially a laminatedretardation optical element having the function of effectivelycompensating for the viewing angle dependency of the optical propertiesof a liquid crystal cell (liquid crystal layer), being in the form of athin film, and capable of effectively preventing lowering of contrastthat is caused by interfacial reflection. The present invention alsorelates to a process of producing the same, and to a liquid crystaldisplay comprising the laminated retardation optical element.

2. Description of Related Art

FIG. 12 is an exploded, diagrammatic perspective view of a conventional,standard liquid crystal display.

As shown in FIG. 12, the conventional liquid crystal display 100comprises a polarizer 102A on the incident side, a polarizer 102B on theemergent side, and a liquid crystal cell 104.

Of these component parts, the polarizers 102A and 102B are soconstructed that they selectively transmit only linearly polarized lighthaving a plane of vibration in a predetermined direction, and arearranged in the cross nicol disposition so that the direction ofvibration of linearly polarized light which the polarizer 102A transmitsis perpendicular to that of vibration of linearly polarized light whichthe polarizer 102B transmits. The liquid crystal cell 104 comprises alarge number of cells corresponding to pixels, and is placed between thepolarizers 102A and 102B.

The case where the liquid crystal cell 104 in the above-described liquidcrystal display 100 is of VA (Vertical Alignment) mode, which a nematicliquid crystal having negative dielectric anisotropy is sealed in aliquid crystal cell (in the figure, the directors of liquid crystallinemolecules are indicated by dotted lines), is now taken as an example.Linearly polarized light that has passed through the polarizer 102A onthe incident side, passes, without undergoing phase shift, through thosecells in the liquid crystal cell 104 that are in the non-driven state,and is blocked by the polarizer 102B on the emergent side. On thecontrary, the linearly polarized light undergoes phase shift when itpasses through those cells in the liquid crystal cell 104 that are inthe driven state, and the light in an amount corresponding to the amountof this phase shift passes through and emerges from the polarizer 102Bon the emergent side. It is therefore possible to display the desiredimage on the emergent-side polarizer 102B side by properly controllingthe driving voltage that is applied to each cell in the liquid crystalcell 104. The liquid crystal display 100 is not limited to the aboveembodiment in which light is transmitted and blocked in theabove-described manner, and there is also a liquid crystal display soconstructed that light emerging from those cells in the liquid crystalcell 104 that are in the non-driven state passes through and emergesfrom the polarizer 102B on the emergent side, and that light emergingfrom those cells that are in the driven state is blocked by thepolarizer 102B on the emergent side.

Discussion is now made on the case where linearly polarized lightpasses, through the non-driven-state cells in the above-described liquidcrystal cell 104 of VA mode. The liquid crystal cell 104 isbirefringent, and its refractive index in the direction of thickness andthat in the direction of plane are different from each other. Therefore,of the linearly polarized light that passed through the polarizer 102Aon the incident side, the light that has entered the liquid crystal cell104 along the normal to it passes through the liquid crystal cell 104without undergoing phase shift, but the light that has slantinglyentered the liquid crystal cell 104 from the direction deviating fromthe normal to it undergoes phase shift while it passes through theliquid crystal cell 104, and becomes elliptically polarized light. Thecause of this phenomenon is that those liquid crystalline molecules thatare vertically aligned in the liquid crystal cell 104 act as a positiveC plate when the cells in the liquid crystal cell 104 of VA mode are inthe non-driven state. It is noted that the amount of phase shift whichlight passing through the liquid crystal cell 104 (transmitted light)undergoes is affected also by the double refractive value of the liquidcrystalline molecules sealed in the liquid crystal cell 104, thethickness of the liquid crystal cell 104, the wavelength of thetransmitted light, and so on.

Owing to the above-described phenomenon, even when the cells in theliquid crystal cell 104 are in the non-driven state and linearlypolarized light is supposed to be transmitted as it is and blocked bythe polarizer 102B on the emergent side, part of the light that hasemerged slantingly from the liquid crystal cell 104 in the directiondeviating from the normal to it is to leak from the polarizer 102B onthe emergent side.

For this reason, the above-described conventional liquid crystal display100 has the problem (viewing angle dependency problem) that the imagequality at the time when an image is viewed slantingly from a positionnot on the normal to the liquid crystal cell 104 is apt to be inferiorto that at the time when the image is viewed from the front of thedisplay.

To eliminate the viewing angle dependency problem with theaforementioned conventional liquid crystal display 100, there have beendeveloped a variety of techniques up to now. One of them is a liquidcrystal display as described, for example, in Patent Document 1 listedbelow. This liquid crystal display uses a retardation optical elementcomprising a retardation layer having a cholesteric structure (aretardation layer having double refractivity), where the retardationoptical element is placed between a liquid crystal cell and a polarizerin order to provide optical compensation.

In the retardation optical element having a cholesteric structure, theselective reflection wavelength given by the equation of “λ=nav·p” (p:the helical pitch in the helical structure consisting of liquidcrystalline molecules; and nav: the mean refractive index of a planeperpendicular to the helical axis) is controlled to be either shorter orlonger than the wavelength of transmitted light, as described, forexample, in Patent Document 2 listed below.

Further, as described, for example, in Patent Document 3 listed below, aliquid crystal display using a retardation optical element comprising aretardation layer (a retardation layer having double refractivity) madefrom a discotic liquid crystal, has also been known as another techniqueof eliminating the above-described viewing angle dependency problem. Inthis liquid crystal display, the retardation optical element is placedbetween a liquid crystal cell and a polarizer in order to provideoptical compensation.

In these retardation optical elements, linearly polarized light that hasslantingly entered the retardation layer from the direction deviatingfrom the normal to it undergoes phase shift, while passing through thisretardation layer, to become elliptically polarized light, as in thecase of the above-described liquid crystal cell of VA mode. The cause ofthis phenomenon is that a cholesteric or discotic liquid crystal acts asa negative C plate. The amount of phase shift which light passingthrough the retardation layer (transmitted light) undergoes is affectedalso by the double refractive value of the liquid crystalline moleculesin the retardation layer, the thickness of the retardation layer, thewavelength of the transmitted light, and so on.

It is therefore possible to eliminate, to a considerable extent, theviewing angle dependency problem with conventional liquid crystaldisplays by the use of the above-described retardation optical element,if the retardation layer contained in the retardation optical element isproperly designed so that a phase difference brought by a liquid crystalcell of VA mode that acts as a positive C plate and a phase differencebrought by the retardation layer acting as a negative C plate, containedin the retardation optical element, are canceled each other.

It is noted that it is possible to eliminate the viewing angledependency problem with liquid crystal displays to a more considerableextent by the combination use of a retardation layer that acts as anegative C plate (i.e., a retardation layer whose refractive indices Nxand Ny in the direction of plane and Nz in the direction of thicknessare in the relationship Nx=Ny>Nz) and a retardation layer that acts asan A plate (i.e., a retardation layer whose refractive indices Nx and Nyin the direction of plane and Nz in the direction of thickness are inthe relationship Nx>Ny=Nz), as described in Patent Document 4 as listedbelow, for example.

In the meantime, the above-described liquid crystal displays comprisingliquid crystal cells of VA mode encompass a liquid crystal displaycomprising a liquid crystal cell of so-called multi-domain VA mode inwhich liquid crystalline molecules are inclined in two or more differentdirections when an electric field is applied. In such a liquid crystaldisplay, it has been known that, if light that enters the liquid crystalcell of multi-domain VA mode is linearly polarized one, lighttransmission is decreased, but that, if light that enters the liquidcrystal cell has been converted to circularly polarized light by a λ/4retardation film, decrease in light transmission can be effectivelyprevented (the following Patent Document 5 and Non-Patent Document 1).

However, in the liquid crystal display as described in Patent Document 5or Non-Patent Document 1, although it is possible to prevent decrease inlight transmission by converting light that enters the liquid crystalcell of multi-domain VA mode into circularly polarized light by a λ/4retardation film, the contrast is, we found, drastically lowered when aretardation layer that acts as a negative C plate is placed between theliquid crystal cell of multi-domain VA mode and the λ/4 retardation filmin order to eliminate the above-described viewing angle dependencyproblem.

In connection with the above-described background art, on the otherhand, the method in which a λ/2 retardation film and a λ/4 retardationfilm are bonded to each other at a predetermined angle, as described,for example, in Patent Document 6 listed below, has been known as amethod for eliminating wavelength dispersion on a λ/4 retardation film.With respect to patterning of a retardation layer, there has been knownsuch a method that a non-patterned λ/4 retardation layer is laminated toa patterned λ/2 retardation layer in order to obtain three-dimensionalimages, as described in Patent Document 7 as listed below, for example.

In addition, we already filed a patent application relating to a filtersubstrate that comprises: a retardation layer (containing a nematicliquid crystal as a main component) having the function of convertinglinearly polarized incident light into circularly polarized light,composed of a λ/2 retardation layer and a λ/4 retardation layer; and acholesteric liquid crystalline filter having the function of selectivelyreflecting the light circularly polarized by the retardation layer (seePatent Document 8 listed below). We also already filed a patentapplication relating to a retardation laminate that comprises apatterned layer of a liquid crystalline material capable of forming anematic layer (see Patent Document 9 listed below).

LIST OF DOCUMENTS

Patent Document 1: Japanese Laid-Open Patent Publication No. 67219/1991

Patent Document 2: Japanese Laid-Open Patent Publication No. 322223/1992

Patent Document 3: Japanese Laid-Open Patent Publication No. 312166/1998

Patent Document 4: Japanese Laid-Open Patent Publication No. 258605/1999

Patent Document 5: Japanese Laid-Open Patent Publication No. 40428/2002

Patent Document 6: Japanese Laid-Open Patent Publication No. 68816/1998

Patent Document 7: Japanese Laid-Open Patent Publication No. 227998/1998

Patent Document 8: Japanese Patent Application No. 342698/2001 (seeJapanese Laid-Open Patent Publication No. 139941/2003)

Patent Document 9: Japanese Patent Application No. 259150/2002 (seeJapanese Laid-Open Patent Publication No. 207641/2003)

Non-Patent Document 1: SID (Society for Information Display) '00, Digestof Tech. Papers, 902 (2000)

SUMMARY OF THE INVENTION

The present invention was accomplished in the light of theaforementioned drawbacks in the related art. An object of the presentinvention is to provide: a laminated retardation optical element thatcan effectively compensate for the viewing angle dependency of theoptical properties of a liquid crystal cell and never decreases contrastand thus never degrades display performance even when placed between aliquid crystal cell and a λ/4 retardation film; a process of producingsuch a laminated retardation optical element; and a liquid crystaldisplay comprising the laminated retardation optical element.

Means of fulfilling the objects

The present invention provides, as a first aspect, a laminatedretardation optical element comprising: an A plate-type retardationlayer that acts as an A plate; and a C plate-type retardation layer thatis optically bonded to the surface of the A plate-type retardation layerand acts as a negative C plate, wherein the A plate-type retardationlayer comprises a cross-linked nematic liquid crystal, and the Cplate-type retardation layer comprises a cross-linked chiral nematic ordiscotic liquid crystal.

In the first aspect of the present invention, the A plate-typeretardation layer is preferably a λ/4 retardation layer having thefunction of brining, to light that passes through this retardationlayer, a phase difference corresponding to a quarter of the wavelengthof the light.

Further, in the first aspect of the present invention, it is preferablethat the laminated retardation optical element further comprises a λ/2retardation layer having the function of bringing, to light that passesthrough this retardation layer, a phase difference corresponding to ahalf of the wavelength of the light, the λ/2 retardation layer beingoptically bonded to the surface of the λ/4 retardation layer serving asthe A plate-type retardation layer, on the side opposite to the Cplate-type retardation layer.

In this case, it is preferable that the λ/2 retardation layer comprisesa cross-linked nematic liquid crystal. It is also preferable that theangle between the axis of phase advance of the λ/4 retardation layerserving as the A plate-type retardation layer and that of the λ/2retardation layer be 60±10 degrees.

Furthermore, in the first aspect of the present invention, it ispreferable that the C plate-type retardation layer has a thickness of 5μm or less.

Furthermore, in the first aspect of the present invention, it ispreferable that: the laminated retardation optical element furthercomprises an additional C plate-type retardation layer that is opticallybonded to the surface of the C plate-type retardation layer on the sideopposite to the A plate-type retardation layer and acts as a negative Cplate; the additional C plate-type retardation layer comprises across-linked chiral nematic or discotic liquid crystal; the totalthickness of the C plate-type retardation layer and the additional Cplate-type retardation layer be 6 μm or more; and the thickness of the Cplate-type retardation layer be nearly equal to that of the additional Cplate-type retardation layer.

Furthermore, in the first aspect of the present invention, it ispreferable that the laminated retardation optical element furthercomprises a polarization layer having the function of controlling thestate of polarization of light that passes through the λ/4 retardationlayer serving as the A plate-type retardation layer.

In this case, it is preferable that the angle between the axis of phaseadvance of the λ/4 retardation layer serving as the A plate-typeretardation layer and the axis of transmission of the polarization layerbe 45±2 degrees (preferably 45 degrees).

Furthermore, in the first aspect of the present invention, it ispreferable that the laminated retardation optical element furthercomprises a polarization layer having the function of controlling thestate of polarization of light that passes through the λ/2 retardationlayer.

In this case, it is preferable that the angle between the axis of phaseadvance of the λ/2 retardation layer and the axis of transmission of thepolarization layer be 15±5 degrees (preferably 15±2 degrees).

Furthermore, in the first aspect of the present invention, it ispreferable that the difference between the mean refractive indices ofthe retardation layers bonded adjacently to each other be 0.05 or less.It is herein preferable that the nematic liquid crystalline componentscontained in the retardation layers bonded adjacently to each other besubstantially the same.

Furthermore, in the first aspect of the present invention, it ispreferable that the A plate-type retardation layer be subjected topatterning to make it into a predetermined pattern. It is alsopreferable that the C plate-type retardation layer be subjected topatterning to make it into a predetermined pattern.

The present invention provides, as a second aspect of the presentinvention, a laminated retardation optical element comprising an Aplate-type retardation layer that acts as an A plate; and a C plate-typeretardation layer that is optically bonded to the surface of the Aplate-type retardation layer and acts as a positive C plate, wherein theA plate-type retardation layer comprises a horizontally-aligned,cross-linked nematic liquid crystal, and the C plate-type retardationlayer comprises a vertically-aligned, cross-linked nematic liquidcrystal.

In the second aspect of the present invention, it is preferable that theC plate-type retardation layer has a thickness of 5 μm or less.

Further, in the second aspect of the present invention, it is preferablethat: the laminated retardation optical element further comprises anadditional C plate-type retardation layer that is optically bonded tothe surface of the C plate-type retardation layer on the side oppositeto the A plate-type retardation layer and acts as a positive C plate;the additional C plate-type retardation layer comprises a cross-linkednematic liquid crystal; the total thickness of the C plate-typeretardation layer and the additional C plate-type retardation layer be 6μm or more; and the thickness of the C plate-type retardation layer benearly equal to that of the additional C plate-type retardation layer.

Furthermore, in the second aspect of the present invention, it ispreferable that the laminated retardation optical element furthercomprises a polarization layer having the function of controlling thestate of polarization of light that passes through the A plate-typeretardation layer.

Furthermore, in the second aspect of the present invention, it ispreferable that the difference between the mean refractive indices ofthe retardation layers bonded adjacently to each other be 0.05 or less.It is herein preferable that the nematic liquid crystalline componentscontained in the retardation layers bonded adjacently to each other besubstantially the same.

Furthermore, in the second aspect of the present invention, it ispreferable that the A plate-type retardation layer be subjected topatterning to make it into a predetermined pattern. It is alsopreferable that the C plate-type retardation layer be subjected topatterning to make it into a predetermined pattern.

The present invention provides, as a third aspect of the presentinvention, a process of producing a laminated retardation opticalelement, comprising the steps of: forming an A plate-type retardationlayer that is in the form of a film and acts as an A plate by applying anematic liquid crystal to an alignment layer and cross-linking theapplied liquid crystal; and forming a C plate-type retardation layerthat is in the form of a film and acts as a negative C plate by applyinga chiral nematic or discotic liquid crystal to the formed A plate-typeretardation layer and cross-linking the applied liquid crystal.

In the third aspect of the present invention, the A plate-typeretardation layer is preferably a λ/4 retardation layers having thefunction of bringing, to light that passes through this retardationlayer, a phase difference corresponding to a quarter of the wavelengthof the light.

Further, in the third aspect of the present invention, it is preferablethat: the process further comprises the step of forming a λ/2retardation layer in the form of a film, having the function ofbringing, to light that passes through this retardation layer, a phasedifference corresponding to a half of the wavelength of the light, byapplying a nematic liquid crystal to the alignment layer andcross-linking the applied liquid crystal; and, in the step of formingthe A plate-type retardation layer, the A plate-type retardation layerbe formed by applying the nematic liquid crystal not to the alignmentlayer but to the λ/2 retardation layer and cross-linking the appliedliquid crystal.

Furthermore, in the third aspect of the present invention, it ispreferable that the process further comprises the step of forming anadditional C plate-type retardation layer that is in the form of a filmand acts as a negative C plate by applying a chiral nematic or discoticliquid crystal to the formed C plate-type retardation layer andcross-linking the applied liquid crystal.

Furthermore, in the third aspect of the present invention, it ispreferable that, in the step of forming the C plate-type retardationlayer on the A plate-type retardation layer, the alignment regulationpower of the surface of the A type-plate retardation layer be used toalign the C plate-type retardation layer. In this case, the alignmentregulation power may be imparted to the surface of the A plate-typeretardation layer by subjecting this surface to rubbing treatment. Theprocess may further comprise the step of forming an additional alignmentlayer on the surface of the A plate-type retardation layer, and, in thestep of forming the C plate-type retardation layer, the alignmentregulation power of the surface of this additional alignment layer maybe used to align the C plate-type retardation layer. The azimuth of thealignment regulation power of the surface of the additional alignmentlayer may be produced by means of rubbing treatment to which theadditional alignment layer is subjected or of optical alignment of theadditional alignment layer.

In the third aspect of the present invention, it is preferable that, inthe step of forming the A plate-type retardation layer on the λ/2retardation layer, the alignment regulation power of the surface of theλ/2 retardation layer be used to align the A plate-type retardationlayer. In this case, the alignment regulation power may be imparted tothe surface of the λ/2 retardation layer by subjecting this surface torubbing treatment. Further, the process may further comprise the step offorming an additional alignment layer on the surface of the λ/2retardation layer, and, in the step of forming the A plate-typeretardation layer, the alignment regulation power of the surface of thisadditional alignment layer may be used to align the A plate-typeretardation layer. The azimuth of the alignment regulation power of thesurface of the additional alignment layer may be produced by means ofrubbing treatment to which the additional alignment layer is subjectedor of optical alignment of the additional alignment layer.

The present invention provides, as a fourth aspect of the presentinvention, a process of producing a laminated retardation opticalelement, comprising the steps of: forming a C plate-type retardationlayer that is in the form of a film, and acts as a negative C plate byapplying a chiral nematic or discotic liquid crystal to an alignmentlayer and cross-linking the applied liquid crystal; and forming an Aplate-type retardation layer that is in the form of a film and acts asan A plate by applying a, nematic liquid crystal to the formed Cplate-type retardation layer and cross-linking the applied liquidcrystal.

In the fourth aspect of the present invention, the A plate-typeretardation layer is preferably a λ/4 retardation layer in the form of afilm, having the function of bringing, to light that passes through thisretardation layer, a phase difference corresponding to a quarter of thewavelength of the light.

The present invention provides, as a fifth aspect of the presentinvention, a liquid crystal display comprising: a liquid crystal cell ofVA mode; a pair of polarizers between which the liquid crystal cell issandwiched; and a laminated retardation optical element (comprising aλ/4 retardation layer and a C plate-type retardation layer) according tothe above-described first aspect, placed between the liquid crystal celland at least one of the polarizers, wherein the laminated retardationoptical element is arranged so that the C plate-type retardation layeris situated on the side close to the liquid crystal cell.

In the fifth aspect of the present invention, it is preferable that theliquid crystal display further comprises an additional λ/4 retardationlayer having the function of bringing, to light that passes through thisretardation layer, a phase difference corresponding to a quarter of thewavelength of the light, placed on the liquid crystal cell on the sideopposite to the laminated retardation optical element.

In the fifth aspect of the present invention, the liquid crystal displayfurther comprises an additional polarization layer having the functionof controlling the state of polarization of light that passes throughthe additional λ/4 retardation layer, placed on the additional λ/4retardation layer on the side opposite to the liquid crystal cell.

In this case, it is preferable that the angle between the axis of phaseadvance of the additional λ/4 retardation layer and the axis oftransmission of the additional polarization layer be 45±2 degrees(preferably 45 degrees).

Further, in the fifth aspect of the present invention, it is preferablethat the angle between the axis of phase advance of the additional λ/4retardation layer and that of the λ/4 retardation layer contained in thelaminated retardation optical element be substantially equal to 90degrees.

In the fifth aspect of the present invention, it is preferable thatliquid crystalline molecules sealed in the liquid crystal cell beinclined in two or more different directions when an electric field isapplied.

The present invention provides, as a sixth aspect of the presentinvention, a liquid crystal display comprising: a liquid crystal cell ofVA mode; a pair of polarizers between which the liquid crystal cell issandwiched; and a laminated retardation optical element (comprising aλ/2 retardation layer, a λ/4 retardation layer and a C plate-typeretardation layer) according to the above-described first aspect, placedbetween the liquid crystal cell and at least one of the polarizers,wherein the laminated retardation optical element is arranged so thatthe C plate-type retardation layer is situated on the side close to theliquid crystal cell.

In the sixth aspect of the present invention, it is preferable that theliquid crystal display further comprises: an additional λ/4 retardationlayer having the function of bringing, to light that passes through thisretardation layer, a phase difference corresponding to a quarter of thewavelength of the light, placed on the liquid crystal cell on the sideopposite to the laminated retardation optical element; and an additionalλ/2 retardation layer having the function of bringing, to light thatpasses through this retardation layer, a phase difference correspondingto a half of the wavelength of the light, placed on the additional λ/4retardation layer on the side opposite to the liquid crystal cell.

In this case, it is preferable that the angle between the axis of phaseadvance of the additional λ/4 retardation layer and that of theadditional λ/2 retardation layer be 60±10 degrees.

Further, in the sixth aspect of the present invention, it is preferablethat the liquid crystal display further comprises an additionalpolarization layer having the function of controlling the state ofpolarization of light that passes through the additional λ/2 retardationlayer, placed on the additional λ/2 retardation layer on the sideopposite to the liquid crystal cell.

In this case, it is preferable that the angle between the axis of phaseadvance of the additional λ/2 retardation layer and the axis oftransmission of the additional polarization layer be 15±5 degrees(preferably 15±2 degrees).

Furthermore, in the sixth aspect of the present invention, it ispreferable that the angle between the axis of phase advance of theadditional λ/2 retardation layer and that of the λ/2 retardation layercontained in the laminated retardation optical element be substantiallyequal to 90 degrees.

In the sixth aspect of the present invention, it is preferable thatliquid crystalline molecules sealed in the liquid crystal cell beinclined in two or more different directions when an electric field isapplied.

Effects of the invention

According to the first aspect of the present invention, an A plate-typeretardation layer that acts as an A plate (preferably a λ/4 retardationlayer), and a C plate-type retardation layer that acts as a negative Cplate, are optically bonded to each other; moreover, the A plate-typeretardation layer and the C plate-type retardation layer comprise across-linked nematic liquid crystal and a cross-linked chiral nematic ordiscotic liquid crystal, respectively. Thus, after light linearlypolarized by a polarizer has been once converted into circularlypolarized light or the like by the A plate-type retardation layer, thephase difference brought by a liquid crystal cell of VA mode can becancelled by the C plate-type retardation layer. For this reason, evenif the liquid crystal cell of VA mode in a liquid crystal display, inwhich the laminated retardation optical element is incorporated, is thatof multi-domain VA mode, the laminated retardation optical element caneffectively compensate for viewing angle dependency. Moreover, accordingto the first aspect of the present invention, since the A plate-typeretardation layer and the C plate-type retardation layer are opticallybonded to each other and, at the same time, are made from cross-linkedliquid crystals, the laminated retardation optical element, a laminateof the two retardation layers, can be made thin. In addition, even whenthe laminated retardation optical element is incorporated in a liquidcrystal display, lowering of contrast that is caused by interfacialreflection that occurs in the laminated retardation optical element canbe effectively prevented.

In the first aspect of the present invention, if a λ/2 retardation layerhaving the function of bringing, to light that passes through thisretardation layer, a phase difference corresponding to a half of thewavelength of the light is optically bonded to the surface of the λ/4retardation layer serving as the A plate-type retardation layer, on theside opposite to the C plate-type retardation layer, compensation forwavelength dispersion on the λ/4 retardation layer is effectivelyprovided by the λ/2 retardation layer. A λ/4 retardation layer thatcovers a wide wave range can thus be obtained as a whole.

In this case, if the λ/2 retardation layer comprises a cross-linkednematic liquid crystal, lowering of contrast can be, prevented moreeffectively.

Further, if the angle between the axis of phase advance of the λ/4retardation layer serving as the A plate-type retardation layer and thatof the λ/2 retardation layer is made 60±10 degrees, compensation forwavelength dispersion on the λ/4 retardation layer can be provided withcertainty.

Furthermore, in the first aspect of the present invention, if thethickness of the C plate-type retardation layer is made 5 μm or less,liquid crystalline molecules in the C plate-type retardation layer canbe well aligned even when only one surface of a liquid crystal layerthat is made into the C plate-type retardation layer is aligned by thealignment regulation power of the surface of the A plate-typeretardation layer (preferably a λ/4 retardation layer) or by that of thesurface of the alignment layer formed on the A plate-type retardationlayer.

Furthermore, in the first aspect of the present invention, if anadditional C plate-type retardation layer that acts as a negative Cplate is optically bonded to the surface of the C plate-type retardationlayer on the side opposite to the A plate-type retardation layer(preferably a λ/4 retardation layer), and if this additional Cplate-type retardation layer is made from a cross-linked chiral nematicor discotic liquid crystal, and if the total thickness of the Cplate-type retardation layer and the additional C plate-type retardationlayer is made 6 μm or more, and if the thickness of the C plate-typeretardation layer is made nearly equal to that of the additional Cplate-type retardation layer, the laminated retardation optical elementcan effectively cope even with the case where the retardation that isbrought by a liquid crystal cell of VA mode and requires compensation bythe C plate-type retardation layer acting as a negative C plate isgreat.

Furthermore, in the first aspect of the present invention, if apolarization layer having the function of controlling the state ofpolarization of light that passes through the λ/4 retardation layerserving as the A plate-type retardation layer is provided, it becomespossible to convert, for example, linearly polarized light to circularlypolarized light, or circularly polarized light to linearly polarizedlight. The desired property of polarizing light can thus be imparted tothe laminated retardation optical element.

Furthermore, in the first aspect of the present invention, if apolarization layer having the function of controlling the state ofpolarization of light that passes through the λ/2 retardation layer usedalong with the λ/4 retardation layer serving as the A plate-typeretardation layer is provided, it becomes possible to convert, forexample, linearly polarized light to circularly polarized light, orcircularly polarized light to linearly polarized light. It is thereforepossible to obtain, from the λ/4 retardation layer and the λ/2retardation layer, a λ/4 retardation layer that covers a wide waverange, and, at the same time, to impart the desired property ofpolarizing light to the laminated retardation optical element.

Furthermore, in the first aspect of the present invention, if thedifference between the mean refractive indices of the retardation layersbonded adjacently to each other is made 0.05 or less, it becomespossible to prevent occurrence of interfacial reflection in thelaminated retardation optical element, and thus to more effectivelyprevent lowering of contrast.

In this case, if the nematic liquid crystalline components contained inthe retardation layers bonded adjacently to each other are madesubstantially the same, the above-described effects can be obtained moresurely.

Furthermore, in the first aspect of the present invention, if the Aplate-type retardation layer (preferably a λ/4 retardation layer) (or aλ/2 retardation layer) is subjected to patterning and made into apredetermined pattern, circularly polarized light that enters a liquidcrystal cell which is driven by the application of an electric field, ina liquid crystal display in which the laminated retardation opticalelement is incorporated, can be made into at least two types of light,for example, right-handed circularly polarized light and left-handedcircularly polarized light. Therefore, by so patterning the A type-plateretardation layer, it becomes possible to produce even a so-calledthree-dimensional display. In addition, if the C plate-type retardationlayer (or the additional C plate-type retardation layer) is subjected topatterning and made into a predetermined pattern equal to the pattern ofthe A plate-type retardation layer (preferably a λ/4 retardation layer)(or a λ/2 retardation layer) or the like, it becomes possible to createat least two retardation areas that are different in viewing angledependency, and thus to provide a laminated retardation optical elementsuitable for the intended use.

According to the second aspect of the present invention, the Cplate-type retardation layer that acts as a positive C plate isoptically bonded to the surface of the A plate-type retardation layerthat acts as a (positive) A plate; moreover, the A plate-typeretardation layer and the C plate-type retardation layer are made from ahorizontally-aligned, cross-linked nematic liquid crystal, and avertically-aligned, cross-linked nematic liquid crystal, respectively.Thus, when the laminated retardation optical element is incorporated ina liquid crystal display, the C plate-type and A plate-type retardationlayers can compensate for the phase difference of light that hasslantingly entered from the direction deviating from the normal to thepolarizers arranged in the cross nicol disposition. For this reason, ina liquid crystal display in which such a laminated retardation opticalelement is incorporated, no light leaks slantingly from the polarizers,and the liquid crystal display can thus have improved viewing anglecharacteristic. Further, according to the second aspect of the presentinvention, since the C plate-type retardation layer and the A plate-typeretardation layer are optically bonded to each other, and, at the sametime, are made from cross-linked liquid crystals, the laminatedretardation optical element, a laminate of the two retardation layers,can be made thin. In addition, even when the laminated retardationoptical element is incorporated in a liquid crystal display, lowering ofcontrast that is caused by interfacial reflection that occurs in thelaminated retardation optical element can be effectively prevented.

In the second aspect of the present invention, if the thickness of the Cplate-type retardation layer is made 5 μm or less, liquid crystallinemolecules in the C plate-type retardation layer can be well aligned evenwhen only one surface of a liquid crystal layer that is made into the Cplate-type retardation layer is aligned by the alignment regulationpower of the surface of the A plate-type retardation layer or by that ofthe surface of the alignment layer formed on the A plate-typeretardation layer.

Further, in the second aspect of the present invention, if an additionalC plate-type retardation layer that acts as a positive C plate isoptically bonded to the surface of the C plate-type retardation layer onthe side opposite to the A plate-type retardation layer, and if thisadditional C plate-type retardation layer is made from a cross-linkednematic liquid crystal, and if the total thickness of the C plate-typeretardation layer and the additional C plate-type retardation layer ismade 6 μm or more, and if the thickness of the C plate-type retardationlayer is made nearly equal to that of the additional C plate-typeretardation layer, the laminated retardation optical element caneffectively cope even with the case where the phase shift that requirescompensation by the C plate-type retardation layer acting as a positiveC plate is in a large amount.

Furthermore, in the second aspect of the present invention, if apolarization layer having the function of controlling the state ofpolarization of light that passes through the A plate-type retardationlayer is provided, the desired property of polarizing light can beimparted to the laminated retardation optical element.

Furthermore, in the second aspect of the present invention, if thedifference between the mean refractive indices of the retardation layersbonded adjacently to each other is made 0.05, or less, it becomespossible to prevent occurrence of interfacial reflection in thelaminated retardation optical element and thus to more effectivelyprevent lowering of contrast.

In this case, if the nematic liquid crystalline components contained inthe retardation layers bonded adjacently to each other are madesubstantially the same, the above-described effects can be obtained moresurely.

Furthermore, in the second aspect of the present invention, if the Aplate-type retardation layer is subjected to patterning and made into apredetermined pattern, circularly polarized light that enters a liquidcrystal cell which is driven by the application of an electric field, ina liquid crystal display in which the laminated retardation opticalelement is incorporated, can be made into at least two different typesof light, for example, right-handed circularly polarized light andleft-handed circularly polarized light. Therefore, by so patterning theA type-plate retardation layer, it becomes possible to produce even aso-called three-dimensional display. In addition, if the C plate-typeretardation layer (or the additional C plate-type retardation layer) issubjected to patterning and made into a predetermined pattern equal tothe pattern of the A plate-type retardation layer or the like, itbecomes possible to create at least two retardation areas that aredifferent in viewing angle dependency, and thus to provide a laminatedretardation optical element suitable for the intended use.

According to the third aspect of the present invention, an A plate-typeretardation layer (preferably a λ/4 retardation layer) that is in theform of a film and acts as an A plate is formed by applying a nematicliquid crystal to an alignment layer and cross-linking the appliedliquid crystal; then, a C plate-type retardation layer that is in theform of a film and acts as a negative C plate is formed by applying achiral nematic or discotic liquid crystal to the formed A plate-typeretardation layer and cross-linking the applied liquid crystal.Therefore, a laminated retardation optical element comprising the Aplate-type retardation layer (preferably a λ/4 retardation layer) andthe C plate-type retardation layer, having the function of effectivelycompensating for the viewing angle dependency of the optical propertiesof a liquid crystal cell, being in the form of a thin film, capable ofpreventing lowering of contrast that is caused by interfacialreflection, can be obtained at high productivity.

In the third aspect of the present invention, the process may furthercomprise the step of forming a λ/2 retardation layer in the form of afilm, having the function of bringing, to light that passes through thisretardation layer, a phase difference corresponding to a half of thewavelength of the light, by applying a nematic liquid crystal to thealignment layer and cross-linking the applied liquid crystal; inaddition, in the step of forming the A plate-type retardation layer(preferably a λ/4 retardation layer), the A plate-type retardation layermay be formed by applying the nematic liquid crystal not to thealignment layer but to the λ/2 retardation layer and cross-linking theapplied liquid crystal. In this case, there can be obtained a laminatedretardation optical element comprising the λ/2 retardation layer, the Aplate-type retardation layer and the C plate-type retardation layer,capable of effectively compensating for wavelength dispersion on the Aplate-type retardation layer (preferably a λ/4 retardation layer).

Further, in the third aspect of the present invention, if an additionalC plate-type retardation layer that is in the form of a film and acts asa negative C plate is formed by applying a chiral nematic or discoticliquid crystal to the formed C plate-type retardation layer andcross-linking the applied liquid crystal, there can be obtained alaminated retardation optical element comprising the C plate-typeretardation layer that is relatively thick and acts as a negative Cplate.

In the third aspect of the present invention, if, in the step of formingthe C plate-type retardation layer on the A plate-type retardation layer(preferably a λ/4 retardation layer), the alignment regulation power ofthe surface of the A type-plate retardation layer is used to align the Cplate-type retardation layer, it is possible to obtain a laminatedretardation optical element without forming an alignment layer on the Aplate-type retardation layer.

In the third aspect of the present invention, if, in the step offorming, the A plate-type retardation layer (preferably a λ/4retardation layer) on the λ/2 retardation layer, the alignmentregulation power of the surface of the λ/2 retardation layer is used toalign the A plate-type retardation layer, it is possible to obtain alaminated retardation optical element without forming an alignment layeron the λ/2 retardation layer.

According to the fourth aspect of the present invention, a C plate-typeretardation layer that is in the form of a film and acts as a negative Cplate is formed by applying a chiral nematic or discotic liquid crystalto an alignment layer and cross-linking the applied liquid crystal;then, an A plate-type retardation layer (preferably a λ/4 retardationlayer) that is in the form of a film and acts as an A plate is formed byapplying a nematic liquid crystal to the formed C plate-type retardationlayer and cross-linking the applied liquid crystal. Therefore, alaminated retardation optical element comprising the A plate-typeretardation layer (preferably a λ/4 retardation layer) and the Cplate-type retardation layer, having the function of effectivelycompensating for the viewing angle dependency of the optical propertiesof a liquid crystal cell, being in the form of a thin film, capable ofpreventing lowering of contrast that is caused by interfacialreflection, can be obtained at high productivity.

According to the fifth aspect of the present invention, the laminatedretardation optical element comprising the λ/4 retardation layer (thelayer having the function of bringing, to light that passes through thisretardation layer, a phase difference corresponding to a quarter of thewavelength of the light) and the C plate-type retardation layer (thelayer acting as a negative C plate) is placed between a liquid crystalcell of VA mode and a polarizer in a liquid crystal display so that theC plate-type retardation layer is situated on the side close to theliquid crystal cell, whereby, of the light in the predetermined state ofpolarization that has entered and/or emerged from the liquid crystalcell, the light that has emerged in the direction deviating from thenormal to the liquid crystal cell is compensated for the state ofpolarization. Therefore, the C plate-type retardation layer can cancelthe phase difference brought by the liquid crystal cell of VA mode. Forthis reason, even if the liquid crystal cell of VA mode is a so-calledliquid crystal cell of multi-domain VA mode, the laminated retardationoptical element can effectively compensate for viewing angle dependency.Further, according to the fifth aspect of the present invention, sincethe λ/4 retardation layer and the C plate-type retardation layercontained in the laminated retardation optical element are opticallybonded, and, at the same time, are made from cross-linked liquidcrystals, the laminated retardation optical element, a laminate of thetwo retardation layers, can be made thin, and lowering of contrast thatis caused by interfacial reflection that occurs in the laminatedretardation optical element can be effectively prevented.

In the fifth aspect of the present invention, if an additional λ/4retardation layer having the function of bringing, to light that passesthrough this retardation layer, a phase difference: corresponding to aquarter of the wavelength of the light is placed on the liquid crystalcell on the side opposite to the laminated retardation optical element,the liquid crystal cell of VA mode and the C plate-type retardationlayer can be sandwiched between a pair of the λ/4 retardation layers.Therefore, it becomes possible to convert, by one of the λ/4 retardationlayers, linearly polarized light into circularly polarized light, and,by the other λ/4 retardation layer, circularly polarized light intolinearly polarized light; there can thus be obtained a liquid crystaldisplay of circularly polarized light VA mode.

In the fifth aspect of the present invention, if an additionalpolarization layer having the function of controlling the state ofpolarization of light that passes through the additional λ/4 retardationlayer is formed on the additional λ/4 retardation layer on the sideopposite to the liquid crystal cell, it becomes possible to convert, forexample, linearly polarized light to circularly polarized light, orcircularly polarized light to linearly polarized light. Therefore, thedesired property of polarizing light can be imparted to the laminatedretardation optical element, and the liquid crystal cell of VA mode canthus be effectively used as an optical shutter.

In the fifth aspect of the present invention, if the angle between theaxis of phase advance of the additional λ/4 retardation layer and thatof the λ/4 retardation layer contained in the laminated retardationoptical element is made substantially equal to 90 degrees, there can beobtained a liquid crystal display with high contrast.

In the fifth aspect of the present invention, in the case that theliquid crystal cell of VA mode is such that liquid crystalline moleculessealed in the liquid crystal cell are inclined in two or more differentdirections when an electric field is applied, it is possible to usecircularly polarized light as light that passes through the liquidcrystal cell, so that the above-described effects can be obtained moreremarkably.

According to the sixth aspect of the present invention, the laminatedretardation optical element comprising the λ/2 retardation layer (thelayer having the function of bringing, to light that passes through thisretardation layer, a phase difference corresponding to a half of thewavelength of the light), the λ/4 retardation layer (the layer havingthe function of bringing, to light that passes through this retardationlayer, a phase difference corresponding to a quarter of the wavelengthof the light), and the C plate-type retardation layer (the layer actingas a negative C plate) is placed between a liquid crystal cell of VAmode and a polarizer in a liquid crystal display so that the Cplate-type retardation layer is situated on the side close to the liquidcrystal cell, whereby, of the light in the predetermined state ofpolarization that has entered and/or emerged from the liquid crystalcell, the light that has emerged in the direction deviating from thenormal to the liquid crystal cell is compensated for the state ofpolarization of light. Therefore, the C plate-type retardation layer cancancel the phase difference brought by the liquid crystal cell of VAmode. For this reason, the laminated retardation optical element caneffectively compensate for viewing angle dependency even when the liquidcrystal cell of VA mode is a so-called liquid crystal cell ofmulti-domain VA mode. Further, according to the fifth aspect of thepresent invention, since the λ/2 retardation layer, the λ/4 retardationlayer and the C plate-type retardation layer contained in the laminatedretardation optical element are optically bonded, and, at the same time,are made from cross-linked liquid crystals, the laminated retardationoptical element, a laminate of these retardation layers, can be madethin, and lowering of contrast that is caused by interfacial reflectionthat occurs in the laminated retardation optical element can beeffectively prevented.

In the sixth aspect of the present invention, if an additional λ/4retardation layer having the function of bringing, to light that passesthrough this retardation layer, a phase difference corresponding to aquarter of the wavelength of the light is placed on the liquid crystalcell on the side opposite to the laminated retardation optical element,and an additional λ/2 retardation layer having the function of bringing,to light that passes through this retardation layer, a phase differencecorresponding to a half of the wavelength of the light is placed on theadditional λ/4 retardation layer on the side opposite to the liquidcrystal cell, the liquid crystal cell of VA mode and the C plate-typeretardation layer are sandwiched between a pair of the λ/4 retardationlayers, whereby a liquid crystal display of circularly polarized lightVA mode can be obtained. In addition, since the λ/2 retardation layereffectively compensates for wavelength dispersion on the λ/4 retardationlayer, there can be obtained, as a whole, a λ/4 retardation layer thatcovers a wide wave range.

In this case, if the angle between the axis of phase advance of theadditional λ/4 retardation layer and that of the additional λ/2retardation layer is made 60±10 degrees, compensation for wavelengthdispersion on the λ/4 retardation layer can be provided with certainty.

Further, in the sixth aspect of the present invention, if an additionalpolarization layer having the function of controlling the state ofpolarization of light that passes through the λ/2 retardation layer usedtogether with the additional λ/4 retardation layer is placed on theadditional λ/2 retardation layer on the side opposite to the liquidcrystal cell, it becomes possible to convert, for example, linearlypolarized light to circularly polarized light, or circularly polarizedlight to linearly polarized light. Therefore, the λ/4 retardation layerand the λ/2 retardation layer can constitute a λ/4 retardation layerthat covers a wide wave range, and, at the same time, the desiredproperty of polarizing light can be imparted to the laminatedretardation optical element. The liquid crystal cell of VA mode can thusbe effectively used as an optical shutter.

Furthermore, in the sixth aspect of the present invention, if the anglebetween the axis of phase advance of the additional λ/2 retardationlayer and that of the λ/2 retardation layer contained in the laminatedretardation optical element is made substantially equal to 90 degrees,there can be obtained a liquid crystal display with high contrast.

In the sixth aspect of the present invention, in the case that theliquid crystal cell of VA mode is such that liquid crystalline moleculessealed in the liquid crystal cell are inclined in two or more differentdirections when an electric field is applied, it is possible to usecircularly polarized light as light that passes through the liquidcrystal cell. The above-described effects can thus be obtained moreremarkably.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded, diagrammatic perspective view showing a liquidcrystal display comprising a laminated retardation optical elementaccording to an embodiment of the present invention;

FIG. 2 is a diagrammatic view for explaining the principle of opticalcompensation that is provided in the liquid crystal display shown inFIG. 1 by the laminated retardation optical element;

FIGS. 3A and 3B are enlarged, diagrammatic perspective views showinglaminated retardation optical elements according to an embodiment of thepresent invention;

FIGS. 4A to 4C are enlarged, diagrammatic perspective views showingmodifications of the laminated retardation optical element according toan embodiment of the present invention;

FIG. 5 is an enlarged, diagrammatic perspective view showing anothermodification of the laminated retardation optical element according toan embodiment of the present invention;

FIGS. 6A and 6B are diagrammatic views illustrating the relationshipbetween the optical axes of the layers contained in the laminatedretardation optical element shown in FIG. 5;

FIGS. 7A and 7B are enlarged, diagrammatic perspective views showingfurther modifications of the laminated retardation optical elementaccording to an embodiment of the present invention;

FIG. 8A to 8C are enlarged, diagrammatic perspective views showing stillfurther modifications of the laminated retardation optical elementaccording to an embodiment of the present invention;

FIG. 9 is a diagrammatic view illustrating a process of producing alaminated retardation optical element according to an embodiment of thepresent invention;

FIG. 10 is a diagrammatic view illustrating another process of producinga laminated retardation optical element according to an embodiment ofthe present invention;

FIG. 11 is a graph showing the relationship between viewing angle andretardation in the laminated retardation optical elements of Example;and

FIG. 12 is an exploded, diagrammatic perspective view showing aconventional liquid crystal display.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

By referring to the accompanying drawings, embodiments of the presentinvention will be described hereinafter.

A liquid crystal display, in which a laminated retardation opticalelement according to an embodiment of the present invention isincorporated, is firstly described with reference to FIG. 1.

As shown in FIG. 1, a liquid crystal display 90 contains a polarizer102A on the incident side, a polarizer 102B on the emergent side, and aliquid crystal cell 104.

Of these component parts, the polarizers 102A and 102B are soconstructed that they selectively transmit only linearly polarized lighthaving a plane of vibration in a predetermined direction, and arearranged in the cross nicol disposition so that the direction ofvibration of linearly polarized light which the polarizer 102A transmitsis perpendicular to that of vibration of linearly polarized light whichthe polarizer 102B transmits. The liquid crystal cell 104 comprises alarge number of cells corresponding to pixels, and is placed between thepolarizers 102A and 102B.

In the liquid crystal display 90, the liquid crystal cell 104 is of VAmode, which a nematic liquid crystal having negative dielectricanisotropy is sealed. Linearly polarized light that has passed throughthe polarizer 102A on the incident side passes, without undergoing phaseshift, through those cells in the liquid crystal cell 104 that are inthe non-driven state, and is blocked by the polarizer 102B on theemergent side. On the contrary, the linearly polarized light undergoesphase shift when it passes through those cells in the liquid crystalcell 104 that are in the driven state, and the light in an amountcorresponding to the amount of this phase shift passes through andemerges from the polarizer 102B on the emergent side. It is thereforepossible to display the desired image on the emergent-side polarizer102B side by properly controlling the driving voltage that is applied toeach cell in the liquid crystal cell 104. The liquid crystal cell 104 ofVA mode is preferably a liquid crystal cell of so-called multi-domain VAmode in which liquid crystalline molecules are inclined in two or moredifferent directions when an electric field is applied.

In the liquid crystal display 90 of such a construction, a laminatedretardation optical element 10 is placed between the polarizer 102A onthe incident side and the liquid crystal cell 104, and a λ/4 retardationfilm 102C, between the polarizer 102B on the emergent side and theliquid crystal cell 104. The laminated retardation optical element 10comprises: a λ/4 retardation layer (an A plate-type retardation layerthat acts as an A plate) 14 having the function of bringing, to lightthat passes through this retardation layer, a phase differencecorresponding to a quarter of the wavelength of the light; and a Cplate-type retardation layer 16 that acts as a negative C plate. Thephase difference (retardation) to be brought by the λ/4 retardationlayer 14 is properly designed according to the wavelength of theobjective light. Specifically, for example, it is designed according toa wavelength freely selected from the visible wave range (400 to 800 nm)in consideration of spectral luminous efficacy and so on. Preferably, asshown in FIG. 1, the laminated retardation optical element 10 is placedso that the λ/4 retardation layer 14 faces to the polarizer 102 on theincident side and that the C plate-type retardation layer 16 faces tothe liquid crystal cell 104. By so placing the laminated retardationoptical element 10, it is possible to effectively obtain the desiredperformance.

Since the laminated retardation optical element 10 contains the λ/4retardation layer 14 as mentioned above, the liquid crystal cell 104 ofVA mode is driven, with the liquid crystal cell 104 sandwiched betweenthe λ/4 retardation layer 14 contained in the laminated retardationoptical element 10 placed on the incident-side polarizer 102A side andthe λ/4 retardation film 102C placed on the emergent-side polarizer 102Bside. The mode of driving liquid crystal display, in which light thatenters the liquid crystal cell 104 of VA mode is circularly polarizedlight, is called circularly polarized light VA mode.

Further, the laminated retardation optical element 10 comprises, asdescribed above, the C plate-type retardation layer 16 that acts as anegative C plate, so that, of the light that enters the liquid crystalcell 104, the light that enters slantingly from the direction deviatingfrom the normal to the liquid crystal cell 104 can be compensated forthe state of polarization of light by the laminated retardation opticalelement 10.

Next, the principle of optical compensation that is provided in theliquid crystal display shown in FIG. 1 by the laminated retardationoptical element 10 will be explained with reference to FIG. 2.

FIG. 2 is a diagrammatic view showing the principle of opticalcompensation that is provided in the case where the liquid crystal cell104 in the liquid crystal display 90 is in the non-driven state andwhere non-polarized light rays W1 and W2 emitted from a light source(not shown in the figure) enter the liquid crystal display 90. In thisfigure, the symbols “←→” and “•” both indicate linearly polarized light,where “←→” indicates linearly polarized light whose field vibrationvector points in the direction of paper plane, and “•” indicateslinearly polarized light whose field vibration vector points in thedirection vertical to the paper plane. The circle with an arrowindicates circularly polarized light.

Referring to FIG. 2, non-polarized light W1 that is emitted from a lightsource (not shown in the figure) and enters the liquid crystal cell 104along the normal to it becomes linearly polarized light because thepolarizer 102A on the incident side absorbs a linearly polarizedcomponent in the direction “•” but transmits the remaining linearlypolarized component in the direction “←→”.

The above linearly polarized light is converted to circularly polarizedlight by the λ/4 retardation layer 14 contained in the laminatedretardation optical element 10, and passes, with its state ofpolarization maintained (circularly polarized light), through the Cplate-type retardation layer 16 in the laminated retardation opticalelement 10 and through the liquid crystal cell 104 to which no electricfield is applied.

The circularly polarized light that has passed through the liquidcrystal cell 104 in this way is converted by the λ/4 retardation film102C into linearly polarized light in the direction “←→” and is blockedby the emergent-side polarizer 102B that transmits only a linearlypolarized component in the direction “•”.

On the other hand, non-polarized light W2 that enters the liquid crystalcell 104 slantingly from the direction deviating from the normal to itbecomes linearly polarized light because the polarizer 102A on theincident side absorbs a linearly polarized component in the direction“•” but transmits a linearly polarized component in the direction “←→”.

The above linearly polarized light is converted to circularly polarizedlight by the λ/4 retardation layer 14 in the laminated retardationoptical element 10. Since the liquid crystal cell 104 of VA mode acts asa positive C plate, light that slantingly enters the liquid crystal cell104 from the direction deviating from the normal to it undergoes phaseshift while passing through the liquid crystal cell 104 to becomeelliptically polarized light.

However, in the liquid crystal display 90 shown in FIG. 2, the amount ofthe phase shift, which occurs due to the liquid crystal cell 104 towhich no electric field is applied being acting as a positive C plate,has been decreased from the light in advance to produce ellipticallypolarized light by the C plate-type retardation layer 16 in thelaminated retardation optical element 10, so that this light returns tocircularly polarized light after passing through the liquid crystal cell104 to which no electric field is applied.

For this reason, the circularly polarized light that has passed throughthe liquid crystal cell 104 in this way is converted by the λ/4retardation film 102C into linearly polarized light in the direction“←→” and is blocked by the emergent-side polarizer 102B that transmitsonly a linearly polarized component in the direction “•”, as in the casewhere non-polarized light W1 enters the liquid crystal cell 104 alongthe normal to it.

As described above, according to the liquid crystal display 90 shown inFIGS. 1 and 2, the laminated retardation optical element 10 is placedbetween the polarizer 102A on the incident side and the liquid crystalcell 104 to optically compensate for the phase shift (retardation)brought by the liquid crystal cell 104, so that it is possible toprevent leakage, from the polarizer 102B on the emergent side, of partof light that has emerged from the liquid crystal cell 104 in thedirection deviating from the normal to it and thus to eliminate theviewing angle dependency problem with the liquid crystal display 90 toimprove viewing angle characteristic.

The liquid crystal display 90 shown in FIGS. 1 and 2 is of transmissiontype which light passes from one side to the other in the direction ofthickness. The present invention is not limited to this, and thelaminated retardation optical element 10 according to theabove-described embodiment may also be incorporated in a liquid crystaldisplay of reflection or semi-reflection (reflection/transmission) type.

Further, in the liquid crystal display 90 shown in FIGS. 1 and 2, thelaminated retardation optical element 10 according to theabove-described embodiment is placed between the liquid crystal cell 104and the polarizer 102A on the incident side. However, the laminatedretardation optical element 10 may be placed between the liquid crystalcell 104 and the polarizer 102B on the emergent side, depending on thestate of optical compensation. Furthermore, the laminated retardationoptical element 10 may also be placed on both sides of the liquidcrystal cell 104 (between the liquid crystal cell 104 and the polarizer102A on the incident side, and between the liquid crystal cell 104 andthe polarizer 102B on the emergent side). The number of the laminatedretardation optical element 10 that is placed between the liquid crystalcell 104 and the polarizer 102A on the incident side or between theliquid crystal cell 104 and the polarizer 102B on the emergent side isnot limited to one, and a plurality of the laminated retardation opticalelements may be placed.

Next, the construction of the laminated retardation optical element 10that is incorporated in the liquid crystal display 90 shown in FIG. 1will be explained with reference to FIG. 3A.

As shown in FIG. 3A, the laminated retardation optical element 10contains: a λ/4 retardation layer (first retardation layer) 14 havingthe function of bringing, to light that passes through this retardationlayer, a phase difference corresponding to a quarter of the wavelengthof the light; and a C plate-type retardation layer (second retardationlayer) 16 that acts as a negative C plate. The λ/4 retardation layer 14and the C plate-type retardation layer 16 are laminated to a transparentsubstrate 12 in the order mentioned, and these two retardation layersare optically bonded to each other.

In the laminated retardation optical element 10 shown in FIG. 3A, theλ/4 retardation layer 14 and the C plate-type retardation layer 16 arelaminated to the transparent substrate 12 in this order. However, it isalso possible to laminate the C plate-type retardation layer 16 and theλ/4 retardation layer 14 to the transparent substrate 12 in this order,as in the laminated retardation optical element 10′ shown in FIG. 3B.

The λ/4 retardation layer 14 comprises as its main component ahorizontally-aligned, cross-linked nematic liquid crystal, and the Cplate-type retardation layer 16 comprises as its main component across-linked chiral nematic liquid crystal (a cross-linked nematicliquid crystal and a cross-linked chiral agent) or cross-linked discoticliquid crystal.

Three-dimensionally cross-linkable liquid crystalline monomers oroligomers can be used as materials for the nematic liquid crystal. Ifany chiral agent is added to the nematic liquid crystal in an amount ofapproximately several to 10%, a chiral nematic liquid crystal(cholesteric liquid crystal) can be obtained. By “three-dimensionalcross-linking” is herein meant that liquid crystalline monomer oroligomer molecules are three-dimensionally polymerized to give a networkstructure. By making the liquid crystalline molecules into such a state,it is possible to optically fix the liquid crystalline molecules withits cholesteric or nematic structure maintained and thus to obtain afilm that is easy to handle as an optical film and is stable at normaltemperatures.

Mixtures of liquid crystalline monomers and chiral compounds asdisclosed, for example, in Japanese Laid-Open Patent Publication No.258638/1995, Published Japanese Translation No. 508882/1998 of PCTInternational Publication for Patent Application, and Japanese Laid-OpenPatent Publications No. 167126/2003 and No. 185827/2003 can be used asthe three-dimensionally cross-linkable monomers. More specifically, itis possible to use liquid crystalline monomers represented by generalchemical formulae (1) to (11) and (11-2). In liquid crystalline monomersrepresented by general chemical formula (11), X is preferably an integerof 2 to 5.

Those compounds represented by general chemical formulae (12) to (14)and (14-2), for example, can be used for the chiral agent. In chiralagents represented by general chemical formulae (12) and (13), X ispreferably an integer of 2 to 12; and in chiral agents represented bygeneral chemical formula (14), X is preferably an integer of 2 to 5. Ingeneral chemical formula (12), R⁴ is hydrogen or methyl group.

On the other hand, it is desirable to use, as the three-dimensionallycross-linkable oligomers, cyclic organopolysiloxane compounds havingcholesteric phases, etc. as disclosed in Japanese Laid-Open PatentPublication No. 165480/1982, for example.

The nematic liquid crystal constituting the λ/4 retardation layer has,because of its nematic-regular structure, double refractivity that makesthe λ/4 retardation layer act as an A plate, so that its refractionindex in the direction of directors of liquid crystalline molecules isdifferent from that in the direction vertical to the directors. Namely,the λ/4 retardation layer 14 has an optical axis extending in thedirection of plane, and if, in the three-dimensional rectangularcoordinate system, the refractive indices of the λ/4 retardation layer14 in the direction of plane are indicated by Nx and Ny and that in thedirection of thickness is indicated by Nz, these indices are in therelationship Nx>Ny=Nz. Therefore, even in the direction of plane, therefractive index (e.g., Nx) in the direction of the directors of liquidcrystalline molecules is different from that (e.g., Ny) in the directionvertical to the directors. It is noted that the refractive index (e.g.,Ny) in the direction of plane, vertical to the directors, is equal tothe refractive index Nz in the direction of thickness.

On the contrary, the cholesteric liquid crystal constituting the Cplate-type retardation layer 16 has, because of its cholesteric-regularstructure, double refractivity that makes the C plate-type retardationlayer act as a negative C plate, so that its refractive index in thedirection of thickness is different from those in the direction ofplane. Namely, the C plate-type retardation layer 16 has an optical axisextending in the direction of plane, and if, in the three-dimensionalrectangular coordinate system, the refractive indices of the Cplate-type retardation layer in the direction of plane are indicated byNx and Ny and that in the direction of thickness is indicated by Nz,these indices are in the relationship Nx=Ny>Nz. Therefore, the Cplate-type retardation layer can shift the phase of circularly polarizedlight that passes through this retardation layer in the directiondeviating from the normal to the laminated retardation optical element10, thereby converting the circularly polarized light into ellipticallypolarized light, and, on the other hand, can shift the phase ofelliptically polarized light that passes through this retardation layerin the direction deviating from the normal, thereby converting theelliptically polarized light into circularly polarized light. It isnoted that the C plate-type retardation layer transmits circularlypolarized light that passes through this retardation layer in thedirection of the normal, without shifting its phase.

The laminated retardation optical element 10 according to thisembodiment is composed of a combination of two retardation layers (theλ/4 retardation layer 14 that acts as an A plate and the C plate-typeretardation layer 16 that acts as a negative C plate) that aredirectionally different in the state of double refraction. Theseretardation layers can cause different phase shifts, and, at the sametime, can very effectively compensate, owing to their synergeticeffects, for changes in optical properties (phase shift, etc.) that arecaused by the liquid crystal cell 104.

Moreover, in the laminated retardation optical element 10, across-linked nematic liquid crystal is used as the main component of theλ/4 retardation layer 14, and a cross-linked chiral nematic liquidcrystal (a cross-linked nematic liquid crystal and a cross-linked chiralagent) or cross-linked discotic liquid crystal, as the main component ofthe C plate-type retardation layer 16. Therefore, the laminatedretardation optical element 10 can have strength, heat resistance andimpact resistance that are satisfactorily high, and can thus be usedeven in a severe environment at 100° C. or higher. In addition, in theprocess of lamination, the λ/4 retardation layer 14 and the C plate-typeretardation layer 16 never mingle with each other, so that it ispossible to obtain excellent optical properties.

In the laminated retardation optical element 10, it is preferable thatthe thickness of the C plate-type retardation layer 16 comprising achiral nematic liquid crystal be 5 μm or less. This is because, if the Cplate-type retardation layer 16 has a thickness of more than 5 μm, ittends to suffer from alignment defect.

Next, modifications of the laminated retardation optical element 10shown in FIG. 3A will be described with reference to FIGS. 4A, 4B and4C.

First of all, a laminated retardation optical element 10A shown in FIG.4A will be described.

The laminated retardation optical element 10A shown in FIG. 4A furthercomprises, in addition to the above-described retardation layers 14 and16, a λ/2 retardation layer 26 having the function of bringing, to lightthat passes through this retardation layer, a phase differencecorresponding to a half of the wavelength of the light. This λ/2retardation layer 26 is optically bonded to the surface of the λ/4retardation layer 14 on the side opposite to the C plate-typeretardation layer 16. The λ/4 retardation layer 14 has the function ofconverting linearly polarized incident light into circularly polarizedlight, or circularly polarized incident light into linearly polarizedlight. On the other hand, the λ/2 retardation layer 26 has the functionof reversing the polarity of polarized light. The phase difference(retardation) to be brought by the λ/2 retardation layer 26 is properlydesigned according to the wavelength of the objective light.Specifically, for example, it is designed according to a wavelengthfreely selected from the visible wave range (400 to 800 nm) inconsideration of spectral luminous efficacy and so on.

The λ/2 retardation layer 26 comprises as its main component ahorizontally aligned, cross-linked nematic liquid crystal.

As shown in FIG. 4C, the λ/4 retardation layer 14 and the λ/2retardation layer 26 are preferably such that the angle θ₁ between theaxis of phase advance L₁ of the λ/4 retardation layer 14 and the axis ofphase advance L₂ of the λ/2 retardation layer 26 is 60±10 degrees. Ifthis angle is so made, a laminate of the λ/4 retardation layer 14 andthe λ/2 retardation layer 26 can constitute a λ/4 retardation layer thatcovers a wide wave range. Specific values for the angle θ₁ between theaxis of phase advance L₁ of the λ/4 retardation layer 14 and the axis ofphase advance L₂ of the λ/2 retardation layer 26 can be selected fromthe desired wave range as described in Patent Document 6 previouslylisted. For example, in the case where the properties on the short waveside are placed above the others, 55±10 degrees is better for the aboveangle than 60±10 degrees.

Thus, according to the laminated retardation optical element 10A shownin FIG. 4A, the influence of the wavelength dispersion properties of theλ/4 retardation layer 14 is minimized by the λ/2 retardation layer 26that is bonded to the λ/4 retardation layer 14 at a predetermined angle,and, as a result, a λ/4 retardation layer that covers a wide wave rangecan be obtained as a whole.

Next, a laminated retardation optical element 10B shown in FIG. 4B willbe described.

The laminated retardation optical element 10B shown in FIG. 4B furthercomprises, in addition to the previously-mentioned two retardationlayers, an additional C plate-type retardation layer 28 that acts as anegative C plate like the C plate-type retardation layer 16. Thisadditional C plate-type retardation layer 28 is optically bonded to thesurface of the C plate-type retardation layer 16 on the side opposite tothe λ/4 retardation layer 14.

The additional C plate-type retardation layer 28 comprises as its maincomponent a cross-linked chiral nematic liquid crystal (a cross-linkednematic liquid crystal and a cross-linked chiral agent) or cross-linkeddiscotic liquid crystal, like the C plate-type retardation layer 16.

It is preferable that both the thickness h1 of the C plate-typeretardation layer 16 and the thickness h2 of the additional C plate-typeretardation layer 28 be 5 μm or less. This is because, if the thicknessh1 of the C plate-type retardation layer 16 and the thickness h2 of theadditional C plate-type retardation layer 28 are more than 5 μm, thesetwo retardation layers tend to suffer alignment defect, and, inaddition, the surface of the C plate-type retardation layer 16 to whichthe C plate-type retardation layer 28 is laminated has decreasedalignment regulation power.

Further, the total thickness (h1+h2) of the C plate-type retardationlayer 16 and the additional C plate-type retardation layer 28 ispreferably 6 μm or more. If the total thickness is so made, there can beeffectively obtained the effects of optical compensation provided by theC plate-type retardation layer 16 and the additional C plate-typeretardation layer 28.

Furthermore, it is preferable that the thickness of the C plate-typeretardation layer 16 be nearly equal to that of the additional Cplate-type retardation layer 28. By controlling so, it becomes easy toproduce the laminated retardation optical element 10, and theproductivity can thus be increased.

Thus, according: to the laminated retardation optical element 10B shownin FIG. 4B, the total thickness of the C plate-type retardation layer 16and the additional C plate-type retardation layer 28 can be made largewith the thickness of each retardation layer maintained small, so thatit is possible to effectively obtain the effects of optical compensationwhile preventing occurrence of alignment defect.

The laminated retardation optical element 10B shown in FIG. 4B is basedon the laminated retardation optical element 10 shown in FIG. 3A.However, the additional C plate-type retardation layer 28 may also beprovided similarly on the laminated retardation optical element 10′ asshown in FIG. 3B or on the laminated retardation optical element 10A asshown in FIG. 4A.

Next, a laminated retardation optical element 20 shown in FIG. 5 will bedescribed.

The laminated retardation optical element 20 shown in FIG. 5 comprises apolarization layer 52, such as a linear polarization layer, bonded tothe surface of the transparent substrate 12 in the laminated retardationoptical element 10 shown in FIG. 3A, on the side opposite to the λ/4retardation layer 14. By so providing the polarization layer 52, it ispossible to impart the desired property of polarizing light, such as theproperty of linearly polarizing light, to the laminated retardationoptical element 20.

In such a laminated retardation optical element 20, the angle θ₂ betweenthe axis of phase advance L₃ of the λ/4 retardation layer 14 and theaxis of transmission L₄ of the polarization layer 52 having the functionof controlling the state of polarization of light that will pass throughthe λ/4 retardation layer 14 is preferably 45±2 degrees, as shown inFIG. 6A.

The laminated retardation optical element 20 shown in FIG. 5 is based onthe laminated retardation optical element 10 as shown in FIG. 3A. Thepolarization layer 52 may also be provided similarly on the laminatedretardation optical element 10′ as shown in FIG. 3B or on the laminatedretardation optical elements 10A and 10B as shown in FIGS. 4A and 4B,respectively.

In the laminated retardation optical element 10A shown in FIG. 4A, ifthe polarization layer 52 is bonded to the surface of the transparentsubstrate 12 on the side opposite to the λ/2 retardation layer 26, theangle θ₃ between the axis of phase advance L₅ of the λ/2 retardationlayer 26 and the axis of transmission L₄ of the polarization layer 52having the function of controlling the state of polarization of lightthat will pass through the λ/2 retardation layer 26 is preferably 15±5degrees, as shown in FIG. 6B.

Next, laminated retardation optical elements 30A and 30B shown in FIGS.7A and 7B, respectively, will be described.

In the laminated retardation optical elements 10, 10′, 10A, 10B and 20shown in FIGS. 3A, 3B, 4A, 4B and 5, respectively, the C plate-typeretardation layer 16 that acts as a negative C plate is used as aretardation layer to be optically bonded to the λ/4 retardation layer 14that acts as an A plate. However, in order to prevent light fromslantingly leaking from the polarizers 102A and 102B that are arrangedin the cross nicol disposition in the liquid crystal display 90 as shownin FIG. 1, a C plate-type retardation layer 16′ that acts as a positiveC plate may be used instead of the C plate-type retardation layer 16.

Specifically, the laminated retardation optical element 30A shown inFIG. 7A comprises: an A plate-type retardation layer 14′ that acts as anA plate; and a C plate-type retardation layer 16′ that acts as apositive C plate. These two retardation layers are optically bonded toeach other. The A plate-type retardation layer 14′ comprises as its maincomponent a horizontally aligned, cross-linked nematic liquid crystal,and, owing to its horizontally-aligned, nematic-regular structure, actsas an A plate having an optical axis (axis of phase delay) extending inthe direction of plane (see the symbol L₇ in FIG. 7B). On the otherhand, the C plate-type retardation layer 16′ comprises as its maincomponent a vertically aligned, cross-linked nematic liquid crystal,and; owing to its vertically-aligned, nematic-regular structure, acts asa positive C plate having an optical axis (axis of phase delay)extending in the direction of thickness (see the symbol L₆ in FIG. 7A).The C plate-type retardation optical layer 16′ that acts as a positive Cplate has an optical axis extending in the direction of thickness, andif, in the three-dimensional rectangular coordinate system, therefractive indices of the C plate-type retardation layer 16′ in thedirection of plane are indicated by Nx and Ny and that in the directionof thickness is indicated by Nz, these refractive indices are in therelationship Nx=Ny<Nz.

In the laminated retardation optical element 30A shown in FIG. 7A, the Aplate-type retardation layer 14′ and the C plate-type retardation layer16′ are laminated to the transparent substrate 12 in this order.However, as in the laminated retardation optical element 30B shown inFIG. 7B, it is also possible to laminate the C plate-type retardationlayer 16′ and the A plate-type retardation layer 14′ to the transparentsubstrate 12 in this order.

Further, in the laminated retardation optical element 30A shown in FIG.7A, the A plate-type retardation layer 14′ to which the C plate-typeretardation layer 16′ is laminated may be any retardation layer as longas it can act as a (positive) A plate, and a variety of retardationlayers, such as a λ/4 retardation layer having the function of bringing,to light that passes through this retardation layer, a phase differencecorresponding to a quarter of the wavelength of the light, or a λ/2retardation layer having the function of bringing, to light that passethrough this retardation layer, a phase difference corresponding to ahalf of the wavelength of the light, can be used for the A plate-typeretardation layer 14′. The phase difference (retardation) that isbrought by the λ/4 retardation layer or the λ/2 retardation layer isproperly designed according to the wavelength of the objective light.Specifically, for example, it is designed according to a wavelength thatis freely selected from the visible wave range (400 to 800 nm) inconsideration of spectral luminous efficacy and so on.

Thus, according to the laminated retardation optical elements 30A and30B shown in FIGS. 7A and 7B, respectively, the C plate-type retardationlayer 16′ acting as a positive C plate is constructed as a mono-axial,birefringent layer that is aligned to have an optical axis (axis ofphase delay) extending in the direction of thickness, and is opticallybonded to the surface of the A plate-type retardation layer 14′ actingas a (positive) A plate, so that if these laminated retardation opticalelements are incorporated in liquid crystal displays 90 as shown in FIG.1, it is possible to compensate, by the C plate-type retardation layer16′ and the A plate-type retardation layer 14′, for the phase shift oflight that enters from the direction deviating from the normal to thepolarizers 102A and 102B arranged in the cross nicol disposition. Forthis reason, in the liquid crystal displays 90 in which the laminatedretardation optical elements 30A and 30B are incorporated, light isprevented from leaking slantingly from the polarizers 102A and 102B; theliquid crystal displays 90 can thus have improved viewing anglecharacteristic.

Next, laminated retardation optical elements 40A, 40B and 40C shown inFIGS. 8A, 8B and 8C, respectively, will be described.

As shown in FIGS. 8A, 8B and 8C, the laminated retardation opticalelements 40A, 40B and 40C are equivalent to such optical elements thatthe λ/4 retardation layer 14, the C plate-type retardation layers 16 and28, and the λ/2 retardation layer 26 that are laminated to thetransparent substrate 12 in the laminated retardation optical elements10, 10′, 10A and 10B as shown in FIGS. 3A, 3B, 4A and 4B, respectively,are subjected to patterning to make them into a predetermined patterncorresponding to the pixel area of the liquid crystal display 90.

In the liquid crystal display 90 in which the laminated retardationoptical element 40A, 40B or 40C is incorporated, if the λ/4 retardationlayer 14 or the λ/2 retardation layer 26 has been patterned as describedabove, circularly polarized light that enters the liquid crystal cell104 which is driven by the application of an electric field can be madeinto at least two types of light, for example, right-handed circularlypolarized light and left-handed circularly polarized light. Therefore,it becomes possible to obtain even a so-called three-dimensionaldisplay.

Further, by patterning the C plate-type retardation layers 16 and 28, itis possible to create at least two retardation areas that are differentin viewing angle dependency. It thus becomes possible to provide alaminated retardation optical element suitable for the intended use.

In the laminated retardation optical elements 10, 10′, 10A and 10B asshown in FIGS. 3A, 3B, 4A and 4B, respectively, in the laminatedretardation optical elements 30A and 30B as shown in FIGS. 7A and 7B,respectively, and in the laminated retardation optical elements 40A, 40Band 40C as shown in FIGS. 8A, 8B, and 8C, respectively, it is preferablethat the difference between the mean refractive indices of each tworetardation layers bonded adjacently to each other (the λ/4 retardationlayer 14, the C plate-type retardation layers 16, 16′ and 28, and theλ/2 retardation layer 26) be 0.05 or less. If this difference is somade, it is possible to effectively prevent occurrence of interfacialreflection in the laminated retardation optical elements 10, 10A, 10B,20, 30A, 30B, 40A, 40B and 40C and thus to effectively prevent loweringof contrast that usually occurs in a liquid crystal display 90 as shownin FIG. 1.

In this case, if the nematic liquid crystalline components contained ineach two retardation layers bonded adjacently to each other (the λ/4retardation layer 14, the C plate-type retardation layers 16, 16′ and28, and the λ/2 retardation layer 26) are made substantially the same,the above-described effects can be obtained more surely.

Next, a process of producing a laminated retardation optical elementaccording to this embodiment, having the above-described construction,will be described with reference to the case where the laminatedretardation optical element 10 shown in FIG. 3A is produced.

First of all, an alignment layer 18 is formed on a transparent substrate12 (FIG. 9 (A)). Inorganic materials such as plates of glass and silica,as well as a variety of resins including polyesters such as celluloseacetate, polyethylene terephthalate (PET) and polyethylene naphthalate(PEN), polyimide, and polyethylene can be used for the transparentsubstrate 12. The alignment layer 18 is laminated to the surface of thetransparent substrate 12, and polymeric films of polyimide, polyamideimide, polyether imide, polyvinyl alcohol and the like can be used forthis alignment layer 18.

The surface 18 a of the alignment layer 18 thus formed is then subjectedto rubbing treatment or the like for alignment (FIG. 9(B)). This surface18 a of the alignment layer 18 subjected to rubbing treatment or thelike becomes such a state that those molecules existing in the vicinityof the surface 18 a are aligned in almost one direction, and has finegrooves that are formed in one direction (the direction indicated by H1in the figure), whereby liquid crystalline molecules that come incontact with this surface 18 a are aligned.

Thereafter, a cross-linkable nematic liquid crystal made from apolymerizable monomer or oligomer, or the like is applied to thealignment layer 18 that has been subjected to rubbing treatment or thelike (FIG. 9(C)). The polymerization of this polymerizable monomer oroligomer, or the like is initiated by the combination use of aphotopolymerization initiator that has been added in advance andultraviolet light that is externally applied, or directly initiated byusing an electron beam, thereby three-dimensionally cross-linking(polymerizing) and solidifying the monomer or oligomer. There is thusformed a λ/4 retardation layer 14 in the form of a film, having thefunction of bringing, to light that passes through this retardationlayer, a phase difference corresponding to a quarter of the wavelengthof the light (FIG. 9(D)). In this process, liquid crystalline moleculesin the λ/4 retardation layer 14 is aligned, in the direction indicatedby H1 in the figure, by the alignment regulation power of the surface 18a of the alignment layer 18. If the entire surface 18 a of the alignmentlayer 18 has been treated so that its alignment regulation power acts insubstantially one direction, it is possible to make the directions ofthe directors of liquid crystalline molecules present on the surface ofthe λ/4 retardation layer that comes in contact with the surface 18 asubstantially the same within this surface. Since the retardation Rbrought by the λ/4 retardation layer 14 is given by the equation R=Δn·d(Δn: double refractive value, d: thickness), it is possible to controlthe retardation R by varying the thickness d of the λ/4 retardationlayer 14. Specifically, for example, in the case where a λ/4 retardationlayer 14 having a double refractive value Δn of 0.1 is required to bringa retardation R of 100 nm, it is proper to make the thickness d of theλ/4 retardation layer 14, 1 μm.

Next, to the λ/4 retardation layer 14 thus formed, a cross-linkablechiral nematic liquid crystal (a cross-linkable nematic liquid crystaland a chiral agent) or discotic liquid crystal made from a polymerizablemonomer or oligomer, or the like is applied (FIG. 9 (F)), and is thenthree-dimensionally cross-linked and solidified by the same technique asthat used for forming the λ/4 retardation layer 14, thereby forming a Cplate-type retardation layer 16 that is in the form of a film and actsas a negative C plate (FIG. 9 (G)). In this process, liquid crystallinemolecules in the C plate-type retardation layer 16 are aligned, in thedirection indicated by H2 in the figure, by the alignment regulationpower of the surface 14 a of the λ/4 retardation layer 14.

In order to decrease, for easy application, the viscosity of thepolymerizable monomers or oligomers to be used to form the λ/4retardation layer 14 and the C plate-type retardation layer 16, they maybe dissolved in proper solvents such as toluene to give coating liquids.If such coating liquids are used, it is necessary to effect the dryingstep of evaporating the solvents before the step of three-dimensionallycross-linking the monomers or oligomers by the application ofultraviolet light or an electron beam.

Thus, there is readily obtained a laminated retardation optical element10 in which the λ/4 retardation layer 14 and the C plate-typeretardation layer 16 are optically bonded to each other. Moreover, sincethe alignment regulation power of the surface 14 a of the λ/4retardation layer 14 can be used to align the liquid crystallinemolecules in the C plate-type retardation layer 16, it is possible toincrease productivity.

In the case where the alignment regulation power of the surface 14 aitself of the λ/4 retardation layer 14 is insufficient, the surface 14 aof the λ/4 retardation layer 14 may be subjected to rubbing treatment toimpart alignment regulation power to this surface.

The liquid crystalline molecules in the C plate-type retardation layer16 may also be aligned in the following manner: as shown in FIGS. 10(D′)to 10(G), after forming an alignment layer 19 on the surface 14 a of theλ/4 retardation layer 14 (FIG. 10(D′), the surface 19 a of thisalignment layer 19 is subjected to rubbing treatment or the like toalign it in the direction indicated by H2 in the figure, and thealignment regulation power of this surface 19 a of the alignment layer19 is used to align the liquid crystalline molecules in the C plate-typeretardation layer 16 (FIGS. 10(E), 10(F) and 10(G)).

In the above description, the azimuth of the alignment regulation powerof the surfaces 18 a and 19 a of the alignment layers 18 and 19 and thatof the alignment regulation power of the surface 14 a of the λ/4retardation layer 14 are produced by means of rubbing treatment. Thepresent invention, however, is not limited to this, and these azimuthsmay also be produced by means of optical alignment. The term “opticalalignment” herein means that linearly polarized light or non-polarizedslant light with a wavelength that causes photo-chemical reaction isapplied to the surface of an optical alignment layer containingoptically active molecules of azobenzene polymers, polyvinyl cinnamate,or the like, thereby imparting anisotropic properties to this surface.In this process, the longer axes of molecules present on the outermostsurface of the optical alignment layer are aligned by the incidentlight, so that liquid crystalline molecules that come in contact withthe molecules on the outermost surface are aligned.

In the above description, after firstly forming the λ/4 retardationlayer 14 comprising a nematic liquid crystal on the alignment layer 18formed on the transparent substrate 12, the C plate-type retardationlayer 16 comprising a chiral nematic or discotic liquid crystal isformed on this λ/4 retardation layer 14. However, the present inventionis not limited to this, and it is also possible to form, after firstlyforming the C plate-type retardation layer 16 comprising a chiralnematic or discotic liquid crystal, the λ/4 retardation layer 14comprising a nematic liquid crystal on this C plate-type retardationlayer 16. In this case, since the nematic liquid crystal is applieddirectly to the C plate-type retardation layer 16, or applied to analignment layer formed on the C plate-type retardation layer 16, it issolidified with the directions of the directors of liquid crystallinemolecules present on the surface of the λ/4 retardation layer 14 on theC plate-type retardation layer 16 side regulated by the alignmentregulation power of the surface of the C plate-type retardation layer 16or of the alignment layer. The other procedures, conditions, etc. inthis production process are basically the same as in the above-describedproduction process, so that detailed descriptions for them are omitted.

Further, in the above description, the process of producing thelaminated retardation optical element 10 shown in FIG. 3A is taken as anexample. This process is also applicable to the production of thelaminated retardation optical elements 10′, 10A, 10B, 20, 30A, 30B, 40A,40B and 40C shown in FIGS. 3B, 4A, 4B, 5, 7A, 7B, 8A, 8B and 8C,respectively.

Specifically, to produce, for example, the laminated retardation opticalelement 10A shown in FIG. 4A, a nematic liquid crystal is applied to analignment layer formed on a transparent substrate 12 and is cross-linkedto form a λ/2 retardation layer 26 in the form of a film, having thefunction of bringing, to light that passes through this retardationlayer, a phase difference corresponding to a half of the wavelength ofthe light; and a nematic liquid crystal is then applied to the formedλ/2 retardation layer 26 and is cross-linked to form a λ/4 retardationlayer 14 in the form of a film, having the function of bringing, tolight that passes through this retardation layer, a phase differencecorresponding to a quarter of the wavelength of the light. In thisprocess, liquid crystalline molecules in the λ/4 retardation layer 14are aligned by the alignment regulation power of the surface of the λ/2retardation layer 26.

In the above process, if the alignment regulation power of the surfaceof the λ/2 retardation layer 26 is insufficient, the surface of the λ/2retardation layer 26 may be subjected to rubbing treatment to impartalignment regulation power to this surface. Alternatively, after formingan alignment layer on the surface of the λ/2 retardation layer 26, thesurface of this alignment layer is aligned by subjecting it to rubbingtreatment or the like, and the alignment regulation power thus impartedto the surface of the alignment layer may be used to align liquidcrystalline molecules in the λ/4 retardation layer 14. Although theazimuth of the alignment regulation power of the surface of the λ/2retardation layer 26 is herein produced by means of rubbing treatment,it may also be produced by means of optical alignment.

To produce the laminated retardation optical element 10B shown in FIG.4B, a chiral nematic liquid crystal (a cross-linkable nematic liquidcrystal and a chiral agent) or discotic liquid crystal is applied to theabove-formed C plate-type retardation layer 16, and is thenthree-dimensionally cross-linked and solidified to form a C plate-typeretardation layer 28 that is in the form of a film and acts as anegative C plate.

It is possible to use the laminated retardation optical element 10 (10A,10B, 20, 30A, 30B, 40A, 40B, 40C) according to the above-describedembodiment by incorporating it in a liquid crystal display 90 as shownin FIG. 1.

In this case, a λ/4 retardation film (additional λ/4 retardation layer)102C having the function of bringing, to light that passes through thisretardation film, a phase difference corresponding to a quarter of thewavelength of the light is placed on a liquid crystal cell 104 on theside opposite to the laminated retardation optical element 10, as shownin FIG. 1. Further, a polarizer (additional polarization layer) 102B isplaced on the λ/4 retardation film 102C on the side opposite to theliquid crystal cell 104. In addition, a λ/2 retardation film (additionalλ/2 retardation layer) 102D having the function of bringing, to lightthat passes through this retardation film, a phase differencecorresponding to a half of the wavelength of the light is placed, ifnecessary, on the λ/4 retardation film 102C on the side opposite to theliquid crystal cell 104 (between the λ/4 retardation film 102C and thepolarizer 102B).

In the case where the λ/2 retardation film 102D is placed, the anglebetween the axis of phase advance of the λ/4 retardation film 102C andthat of the λ/2 retardation film 102D is preferably 60±10 degrees.Specific values for the angle between the axis of phase advance of theλ/4 retardation film 102C and that of the λ/2 retardation film 102D canbe selected from the desired wave range as described in Patent Document6 as listed previously. For example, in the case where the properties onthe short wave side are placed above the others, 55±10 degrees is betterfor the above angle than 60±10 degrees. Further, the angle between theaxis of phase advance of the λ/4 retardation film 102C and the axis oftransmission of the polarizer 102B is preferably 45±2 degrees.Furthermore, the angle between the axis of phase advance of the λ/2retardation film 102D and the axis of transmission of the polarizer 102Bis preferably 15±5 degrees.

In the above-described liquid crystal display 90, it is preferable thatthe angle between the axis of phase advance of the λ/4 retardation film102C and that of the λ/4 retardation layer 14 contained in the laminatedretardation optical element 10 be substantially equal to 90 degrees.Further, in the case where the λ/2 retardation film 102D is placed andthe laminated retardation optical element 10A comprising the λ/2retardation layer 26 as shown in FIG. 4A is used as the laminatedretardation optical element 10, it is preferable that the angle betweenthe axis of phase advance of the λ/2 retardation film 102D and that ofthe λ/2 retardation layer 26 contained in the laminated retardationoptical element 10A be substantially equal to 90 degrees. If theseangles are so controlled, incomplete circular polarization of light thathas firstly passed through the λ/4 retardation layer 14 or λ/2retardation layer 26 is cancelled by the subsequent λ/4 retardation film102C or λ/2 retardation film 102D; it is therefore possible to increasecontrast.

EXAMPLE

The aforementioned embodiments of the invention will now be explainedmore specifically by referring to the following Example, in which theproduction of the laminated retardation optical element 10 shown in FIG.3A is taken as an example.

Example

A toluene solution (nematic liquid crystal solution) was prepared bydissolving a monomer containing, in its molecule, polymerizableacrylates at both ends and spacers between mesogen existing at thecenter and the acrylates, having a nematic-isotropic transitiontemperature of 110° C. (a monomer having a molecular structurerepresented by the above chemical formula (11)). To this nematic liquidcrystal solution, a photopolymerization initiator (“Irgacure® 907”available from Ciba Specialty Chemicals K.K., Japan) was added in anamount of 5% by weight of the above-described monomer.

On the other hand, a transparent glass substrate was spin-coated withpolyimide (“Optomer® AL1254” manufactured by JSR Corporation, Japan)dissolved in a solvent. After drying, a film of the polyimide (filmthickness: 0.1 μm) was formed at 200° C., and was rubbed in onedirection so that it could function as an alignment layer.

The glass substrate coated with the alignment layer was set in aspin-coater, and was spin-coated with the above-described nematic liquidcrystal solution.

The toluene contained in the nematic liquid crystal solution was thenevaporated at 80° C. to form a coating film. Ultraviolet light wasapplied to this coating film, and with radicals thus released from thephotopolymerization initiator contained in the coating film, theacrylates in the monomer molecules were three-dimensionally crosslinkedand solidified (polymerized) to give a layer (an A plate-typeretardation layer acting as an A plate) in the form of a film, having anematic-regular structure.

Next, a toluene solution (chiral nematic liquid crystal solution) wasprepared by dissolving 90 parts of the above-described monomer (havingthe molecular structure represented by the above chemical formula (11))and 10 parts of a chiral agent comprising acrylates at the both ends ofits molecule (having the molecular structure represented by the abovechemical formula (14)). The above A plate-type retardation layer wasspin-coated with this chiral nematic liquid crystal solution.

The toluene contained in the chiral nematic liquid crystal solution wasthen evaporated at 80° C. to form a coating film. Ultraviolet light wasapplied to this coating film, and with radicals thus released from thephotopolymerization initiator contained in the coating film, theacrylates in the monomer molecules were three-dimensionally crosslinkedand solidified (polymerized) to give a layer (a C plate-type retardationlayer acting as a negative C plate) in the form of a film, having acholesteric-regular structure.

Thus, there was finally obtained a laminated retardation optical elementin which the layer (an A plate-type retardation layer acting as an Aplate) having a nematic-regular structure and the layer (a C plate-typeretardation layer acting as a negative C plate) having acholesteric-regular structure were adjacently laminated to each other.

In this Example, three different laminated retardation optical elementsX, Y and Z were produced by varying the thickness of the A plate-typeretardation layer and that of the C plate-type retardation layer.Namely, a laminated retardation optical element X with a total filmthickness of 4 μm was produced, where the thickness of the A plate-typeretardation layer was made 2 μm and that of the C plate-type retardationlayer was made 2 μm. Further, a laminated retardation optical element Ywith a total film thickness of 2.5 μm was produced, where the thicknessof the A plate-type retardation layer was made 0.5 μm and that of the Cplate-type retardation layer was made 2 μm. Furthermore, a laminatedretardation optical element Z with a total film thickness of 3.2 μm wasproduced, where the thickness of the A plate-type retardation layer wasmade 1.2 μm and that of the C plate-type retardation layer was made 2μm.

Comparative Example

A laminated retardation optical element of Comparative Example wasproduced by the combination use of an oriented, norbornene resin filmacting as the λ/4 retardation layer and three TAC films acting asnegative C plates, prepared by bonding the films with an adhesive agent.

(Results of Evaluation)

The effects, on optical compensation, of the laminated retardationoptical elements X, Y and Z of Example produced in the above-describedmanner were evaluated. Specifically, the relationship between viewingangle and retardation was obtained by the use of an automatic doublerefractivity measuring apparatus (“KOBRA® 21ADH” manufactured by OjiKeisoku Kiki Kabushiki Kaisha, Japan).

FIG. 11 is a graph showing the relationship between viewing angle andretardation in the laminated retardation optical elements X, Y and Z.FIG. 11 plots viewing angle (°) as the abscissa and retardation (nm) asthe ordinate.

As is clear from FIG. 11, all of the laminated retardation opticalelements X, Y and Z of Example have optical properties that are equal tothe sum of the optical properties of the A plate and those of the Cplate.

Further, the laminated retardation optical elements X, Y and Z ofExample were respectively incorporated in liquid crystal displays asshown in FIG. 1, and contrast was observed. As a result, the contrastwas found to be satisfactorily high as compared with the case where thelaminated retardation optical element of Comparative Example wasincorporated.

All of the laminated retardation optical elements X, Y and Z of Examplewere produced by laminating the C plate-type retardation layer to the Aplate-type retardation layer by means of direct coating, so that therewas no need to provide a transparent substrate or the like between theseretardation layers. Thinning of laminated retardation optical elementswas thus accomplished.

1. A liquid crystal display comprising: a liquid crystal cell of VAmode; a pair of polarizers between which the liquid crystal cell issandwiched; a laminated retardation optical element placed between theliquid crystal cell and at least one of the polarizers, the laminatedretardation optical element comprising: an A plate-type retardationlayer that acts as an A plate; and a C plate-type retardation layer thatis optically bonded to a surface of the A plate-type retardation layerand acts as a negative C plate, wherein the C plate-type retardationlayer has a thickness of 5 μm or less; wherein: the C plate-typeretardation layer comprises a cross-linked chiral nematic liquidcrystal; and the A plate-type retardation layer is a λ/4 retardationlayer having a function of bringing, to light that passes through thisretardation layer, a phase difference corresponding to a quarter of awavelength of the light; wherein the laminated retardation opticalelement is arranged so that the C plate-type retardation layer issituated on a side close to the liquid crystal cell; an additional λ/4retardation layer having a function of bringing, to light that passesthrough this retardation layer, a phase difference corresponding to aquarter of a wavelength of the light, placed on the liquid crystal cellon a side opposite to the laminated retardation optical element; and anadditional polarization layer having a function of controlling a stateof polarization of light that passes through the additional λ/4retardation layer, placed on the additional λ/4 retardation layer on aside opposite to the liquid crystal cell; wherein an angle between anaxis of phase advance of the additional λ/4 retardation layer and anaxis of transmission of the additional polarization layer is 45±2degrees.
 2. The liquid crystal display according to claim 1, thelaminated retardation optical element further comprising a λ/2retardation layer having a function of bringing, to light that passesthrough this retardation layer, a phase difference corresponding to ahalf of a wavelength of the light, the λ/2 retardation layer beingoptically bonded to a surface of the λ/4 retardation layer serving asthe A plate-type retardation layer, on a side opposite to the Cplate-type retardation layer.
 3. The liquid crystal display according toclaim 2, wherein the λ/2 retardation layer comprises a cross-linkednematic liquid crystal.
 4. The liquid crystal display according to claim2, wherein an angle between an axis of phase advance of the λ/4retardation layer serving as the A plate-type retardation layer and thatof the λ/2 retardation layer is 60±10 degrees.
 5. The liquid crystaldisplay according to claim 2, further comprising a polarization layerhaving a function of controlling a state of polarization of light thatpasses through the λ/2 retardation layer.
 6. The liquid crystal displayaccording to claim 5, wherein an angle between an axis of phase advanceof the λ/2 retardation layer and an axis of transmission of thepolarization layer is 15±5 degrees.
 7. The liquid crystal displayaccording to claim 1, the laminated retardation optical element furthercomprising an additional C plate-type retardation layer that isoptically bonded to a surface of the C plate-type retardation layer on aside opposite to the A plate-type retardation layer and acts as anegative C plate, wherein the additional C plate-type retardation layercomprises a cross-linked chiral nematic or discotic liquid crystal, atotal thickness of the C plate-type retardation layer and the additionalC plate-type retardation layer is 6 μm or more, and a thickness of the Cplate-type retardation layer is nearly equal to that of the additional Cplate-type retardation layer.
 8. The liquid crystal display according toclaim 1, the laminated retardation optical element further comprising apolarization layer having a function of controlling a state ofpolarization of light that passes through the λ/4 retardation layerserving as the A plate-type retardation layer.
 9. The liquid crystaldisplay according to claim 8, wherein an angle between an axis of phaseadvance of the λ/4 retardation layer serving as the A plate-typeretardation layer and an axis of transmission of the polarization layeris 45±2 degrees.
 10. The liquid crystal display according to claim 1,wherein nematic liquid crystalline components contained in theretardation layers bonded adjacently to each other are substantially thesame.
 11. The liquid crystal display according to claim 1, wherein the Aplate-type retardation layer is subjected to patterning to make it intoa predetermined pattern.
 12. The liquid crystal display according toclaim 1, wherein the C plate-type retardation layer is subjected topatterning to make it into a predetermined pattern.
 13. The liquidcrystal display according to claim 1, wherein an angle between an axisof phase advance of the additional λ/4 retardation layer and that of theλ/4 retardation layer contained in the laminated retardation opticalelement is substantially equal to 90 degrees.
 14. The liquid crystaldisplay according to claim 1, wherein liquid crystalline moleculessealed in the liquid crystal cell are inclined in two or more differentdirections when an electric field is applied.